An attempt is being made to elucidate the factors responsible for the initiation of aggressive behavior in female mice following parturition (post-partum aggression). It has been found that such aggressive behavior can be induced by artificially terminating pregnancy via hysterectomy, a manipulation which also induces lactation. Stimulation of the nipples by the pups is apparently important in triggering post-partum aggression since mice thelectomized during pregnancy or within the first 24 hours post-partum will not display aggressive behavior whereas animals thelectomized following 48 hours of suckling will engage in aggression. Therefore, mice require between 24 and 48 hours of nipple stimulation for the display of aggression. Finally, virgin female mice will exhibit aggressive behavior following daily treatment with estradiol benzoate and progesterone, a hormonal regimen which has been shown to induce nipple development.